1. Field of the Invention
This invention relates to the allocation of transmit power in the downlink of a distributed MIMO cellular radio network to achieve generally equal throughput for each mobile terminal whilst generally maximizing throughput for the network.
2. Description of the Related Art
It is often desirable to provide a service uniformly within the coverage area of a cellular wireless network, despite variations in propagation conditions experienced by users in different locations. Conventionally this may be achieved by a scheduling technique in which the base station of the network spends a greater proportion of time transmitting to users in areas of poor reception and compensates by using a higher level of modulation (and hence higher data rate) for users with better reception.
This prior art technique is equally applicable to SISO (single input, single output) systems, in which a single spatial mode of the wireless channel is employed for communication between the base station and each mobile terminal, and to MIMO (multiple input, multiple output) systems, in which multiple spatial modes of the wireless channel are employed for communication between the base station and each mobile terminal.
A further aspect of a MIMO system concerns the allocation of the transmitter power between the spatial modes of the channel between the base station and an individual mobile terminal. In such a system, it is desirable that the power be allocated so as to maximise the capacity of the MIMO channel. Where multiple spatial modes of the wireless channel are employed for communication between the base station and each mobile terminal, a technique known as “water-pouring” may be employed, in which the power of the transmitter is allocated so as to maximise the capacity of the channel as a whole. Although this technique results in significantly improved capacity, it often requires that all of the transmitter power be allocated to one of the spatial modes. It can therefore only be used if all of the modes of the channel are used to support communication with a single mobile terminal, as in conventional MIMO systems. If it were to be employed in a system where, for example, each mode of the channel were to be used to support communication with a different mobile terminal, then those mobile terminals reliant upon modes with no power allocation would find themselves effectively outside the coverage of the base station, which typically is undesirable. A MIMO system in which the spatial modes serve more than one mobile station is known as a distributed MIMO system. It is also sometimes known as Virtual MIMO or Cooperative MIMO.